This invention relates to vacuum pumps and, more particularly, to molecular drag vacuum pump structures.
Molecular drag vacuum pumps produce pumping action by momentum transfer from a fast-moving surface directly to gas molecules. A typical molecular drag vacuum pump includes a rotating element, or rotor, and a stationary element, or stator. The stator is provided with a channel between an inlet and an outlet. Collisions of gas molecules with the moving rotor cause gas in the channel to be pumped from the inlet to the outlet. In order to obtain a significant pressure ratio, the pressure should be relatively low, i.e., molecular flow conditions at least at the pump inlet, and the rotor velocity should approach the average velocity of the gas molecules.
Molecular drag vacuum pumps may be utilized in combination with other types of vacuum pumps. A vacuum pump utilizing turbomolecular vacuum pumping stages and molecular drag stages is disclosed in U.S. Pat. No. 5,238,362, issued Aug. 24, 1993 to Casaro et al.
One known type of molecular drag vacuum pump is the so-called Siegbahn-type pump. The Siegbahn-type pump is characterized by a rotor in the form of a disk and stator having a spiral channel that extends from the outer periphery of the disk toward the hub or center portion of the rotating disk. In the conventional Siegbahn-type pump, the inlet is at the outer periphery of the disk and the outlet or exhaust is located near the center of the disk. This arrangement was utilized because it is better for both pumping speed and compression ratio to have a high relative velocity between the rotor and the stationary pumping channel. The highest velocity of a rotating disk is achieved at its outer periphery.
In some applications, pumping speed and compression ratio are less important than small size and light weight. Accordingly, there is a need for improved molecular drag vacuum pump configurations.
According to a first aspect of the invention, vacuum pumping apparatus is provided. The vacuum pumping apparatus comprises a rotor, a motor for rotating the rotor about an axis of rotation, a stator mounted in proximity to the rotor, the stator including at least one spiral channel having an open side facing said rotor, and a housing enclosing the rotor and the stator. The housing defines an inlet in fluid communication with the spiral channel at the inner portion of the stator. Gas is pumped outwardly with respect to the axis of rotation through the spiral channel as the motor rotates the rotor.
The rotor may comprise a disk. The spiral channel may decrease in cross-sectional area from larger at the inner portion of the stator to smaller at the outer portion of the stator.
In one embodiment, the stator comprises two or more spiral channels coupled in parallel between the inlet and the outer portion of the stator. Each of the two or more spiral channels may decrease in cross-sectional area from larger at the inner portion of the stator to smaller at the outer portion of the stator.
The at least one spiral channel may define a first vacuum pumping stage on a first side of the rotor. The apparatus may further comprise a second vacuum pumping stage on a second side of the rotor and a series connection between the first and second vacuum pumping stages. In a first configuration, the second vacuum pumping stage comprises a molecular drag vacuum pumping stage having a second stage stator that defines at least one spiral channel. In a second configuration, the second vacuum pumping stage comprises a molecular drag vacuum pumping stage having a second stage rotor that defines two or more channels. The two or more channels may be connected in series or in parallel. The two or more channels may have spiral configurations or concentric circular configurations. In a third configuration, the second vacuum pumping stage comprises at least one regenerative vacuum pumping stage. In the regenerative vacuum pumping stage, the rotor may be provided with radial ribs or blades which define cavities.
According to another feature of the invention, the motor may comprise a pancake-type motor having a generally disk-shaped rotor. The disk-shaped rotor of the pancake-type motor may function as the rotor of a third vacuum pumping stage.
According to another aspect of the invention, vacuum pumping apparatus is provided. The vacuum pumping apparatus comprises a rotor in the form of a disk, a motor for rotating the disk about an axis of rotation, a first stator mounted in proximity to a first side of the disk, and a second stator mounted in proximity to a second side of the disk. The first stator defines a first channel configuration between an inner portion of the first stator near the axis and an outer portion of the first stator. The first channel configuration has an inlet at or near the inner portion of the first stator and comprises at least one spiral channel having a cross section that decreases from larger near the inlet to smaller near the outer portion of the first stator. The second stator defines a second channel configuration between the outer portion of the second stator and the inner portion of the second stator near the axis. The second channel configuration has an outlet at or near the inner portion of the second stator. The vacuum pumping apparatus further comprises a series connection between the first channel configuration and the second channel configuration. Gas is pumped outwardly with respect to the axis of rotation through the first channel configuration as the motor rotates the disk.
According to a further aspect of the invention, vacuum pumping apparatus is provided. The vacuum pumping apparatus comprises a first vacuum pumping stage having an inlet, a second vacuum pumping stage having an outlet, and a conduit for connecting the first and second vacuum pumping stages in series. The first vacuum pumping stage comprises a rotor, a motor for rotating the rotor about an axis of rotation, and a stator having at least one spiral channel between an inner portion of the stator and an outer portion of the stator. The inlet is connected to the at least one spiral channel at the inner portion of the stator. Gas is pumped outwardly with respect to the axis of rotation through the at least one spiral channel as the motor rotates the rotor.